I did a little looking around I found 9.00 20's they are hard to find and expensive, about $330 - 425 each. 8.25 are a little cheaper, I found them @ $277, delivered to my zip 97502.
https://www.ebay.com/itm/38358484414...Bk9SR-j0rLGPYg
10.00 20's are still common @ ~$380
I did find a interesting tire, that could be used on the front. It's a 3 rib, 7.50 20, implement tire (like a tractor tire) that could make steering a a lot easier for $154 ea.
I couldn't find anything smaller than the 8.25's for truck tires.
My '35 has 7.00 20's on the front and dual 6.50's on the back.